Continuous application and decompression of test patterns and selective compaction of test responses

ABSTRACT

A method for applying test patterns to scan chains in a circuit-under-test. The method includes providing a compressed test pattern of bits; decompressing the compressed test pattern into a decompressed test pattern of bits as the compressed test pattern is being provided; and applying the decompressed test pattern to scan chains of the circuit-under-test. The actions of providing the compressed test pattern, decompressing the compressed test pattern, and applying the decompressed pattern are performed synchronously at the same or different clock rates, depending on the way in which the decompressed bits are to be generated. A circuit that performs the decompression includes a decompressor such as a linear finite state machine adapted to receive a compressed test pattern of bits. The decompressor decompresses the test pattern into a decompressed test pattern of bits as the compressed test pattern is being received.

RELATED APPLICATION DATA

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/891,498, filed on Sep. 27, 2010 now U.S. Pat. No. 8,108,743,which is a continuation of U.S. patent application Ser. No. 12/396,377,filed Mar. 2, 2009, now U.S. Pat. No. 7,805,649, which is a continuationof U.S. patent application Ser. No. 10/973,522, filed Oct. 25, 2004, nowU.S. Pat. No. 7,500,163, which is a continuation of U.S. patentapplication Ser. No. 10/354,576, filed Jan. 29, 2003, now U.S. Pat. No.6,829,740, which is a continuation of U.S. patent application Ser. No.09/619,988, filed Jul. 20, 2000, now U.S. Pat. No. 6,557,129, whichclaims the benefit of U.S. Provisional Application No. 60/167,136, filedNov. 23, 1999, all of which are hereby incorporated herein by reference.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/352,994, filed Jan. 13, 2009, now U.S. Pat. No.7,877,656, which is a continuation of U.S. patent application Ser. No.10/354,633, filed Jan. 29, 2003, now U.S. Pat. No. 7,478,296, which is acontinuation of U.S. patent application Ser. No. 09/620,021, filed Jul.20, 2000, now U.S. Pat. No. 7,493,540, which claims the benefit of U.S.Provisional Application No. 60/167,131, filed Nov. 23, 1999, all ofwhich are hereby incorporated herein by reference.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/983,815, filed on Jan. 3, 2011 now abandoned,which is a continuation of U.S. patent application Ser. No. 12/402,880,filed Mar. 12, 2009, now U.S. Pat. No. 7,865,794, which is acontinuation of U.S. patent application Ser. No. 11/502,655, filed Aug.11, 2006, now U.S. Pat. No. 7,506,232, which is a continuation of U.S.patent application Ser. No. 10/736,966, filed Dec. 15, 2003, now U.S.Pat. No. 7,093,175, which is a continuation of U.S. patent applicationSer. No. 09/713,664, filed Nov. 15, 2000, now U.S. Pat. No. 6,684,358,which claims the benefit of U.S. Provisional Application No. 60/167,137,filed Nov. 23, 1999, all of which are hereby incorporated herein byreference.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 11/894,393, filed on Aug. 20, 2007 now U.S. Pat.No. 8,024,387, which is a continuation of U.S. patent application Ser.No. 10/781,031, filed Feb. 17, 2004, now U.S. Pat. No. 7,260,591, whichis a continuation U.S. patent application Ser. No. 10/346,699, filedJan. 16, 2003, now U.S. Pat. No. 6,708,192, which is a continuation ofU.S. patent application Ser. No. 09/957,701, filed Sep. 18, 2001, nowU.S. Pat. No. 6,539,409, which is a continuation of U.S. patentapplication Ser. No. 09/620,023, filed Jul. 20, 2000, now U.S. Pat. No.6,353,842, which claims the benefit of U.S. Provisional Application No.60/167,445, filed Nov. 23, 1999, all of which are hereby incorporatedherein by reference.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/405,409, filed on Mar. 17, 2009 now U.S. Pat.No. 7,900,104, which is a continuation of Ser. No. 11/523,111 filed Sep.18, 2006, now U.S. Pat. No. 7,509,546, which is a continuation of U.S.patent application Ser. No. 10/355,941 filed Jan. 31, 2003, now U.S.Pat. No. 7,111,209, which is a continuation of U.S. patent applicationSer. No. 09/947,160 filed Sep. 4, 2001, now U.S. Pat. No. 6,543,020,which is a continuation of U.S. patent application Ser. No. 09/619,985filed Jul. 20, 2000, now U.S. Pat. No. 6,327,687, which claims thebenefit of U.S. Provisional Application No. 60/167,446 filed Nov. 23,1999, all of which are hereby incorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to testing of integrated circuits and,more particularly, to the generation and application of test data in theform of patterns, or vectors, to scan chains within acircuit-under-test. This invention also relates generally to testing ofintegrated circuits and more particularly relates to compaction of testresponses used in testing for faults in integrated circuits.

BACKGROUND

As integrated circuits are produced with greater and greater levels ofcircuit density, efficient testing schemes that guarantee very highfault coverage while minimizing test costs and chip area overhead havebecome essential. However, as the complexity of circuits continues toincrease, high fault coverage of several types of fault models becomesmore difficult to achieve with traditional testing paradigms. Thisdifficulty arises for several reasons. First, larger integrated circuitshave a very high and still increasing logic-to-pin ratio that creates atest data transfer bottleneck at the chip pins. Second, larger circuitsrequire a prohibitively large volume of test data that must be thenstored in external testing equipment. Third, applying the test data to alarge circuit requires an increasingly long test application time. Andfourth, present external testing equipment is unable to test such largercircuits at their speed of operation.

Integrated circuits are presently tested using a number of structureddesign for testability (DFT) techniques. These techniques rest on thegeneral concept of making all or some state variables (memory elementslike flip-flops and latches) directly controllable and observable. Ifthis can be arranged, a circuit can be treated, as far as testing ofcombinational faults is concerned, as a combinational network. Themost-often used DFT methodology is based on scan chains. It assumes thatduring testing, all (or almost all) memory elements are connected intoone or more shift registers, as shown in the U.S. Pat. No. 4,503,537. Acircuit that has been designed for test has two modes of operation: anormal mode and a test, or scan, mode. In the normal mode, the memoryelements perform their regular functions. In the scan mode, the memoryelements become scan cells that are connected to form a number of shiftregisters called scan chains. These scan chains are used to shift a setof test patterns into the circuit and to shift out circuit, or test,responses to the test patterns. The test responses are then compared tofault-free responses to determine if the circuit-under-test (CUT) worksproperly.

Scan design methodology has gained widespread adoption by virtue of itssimple automatic test pattern generation (ATPG) and silicon debuggingcapabilities. Today, ATPG software tools are so efficient that it ispossible to generate test sets (a collection of test patterns) thatguarantee almost complete fault coverage of several types of faultmodels including stuck-at, transition, path delay faults, and bridgingfaults. Typically, when a particular potential fault in a circuit istargeted by an ATPG tool, only a small number of scan cells, e.g., 2-5%,must be specified to detect the particular fault (deterministicallyspecified cells). The remaining scan cells in the scan chains are filledwith random binary values (randomly specified cells). This way thepattern is fully specified, more likely to detect some additionalfaults, and can be stored on a tester.

Because of the random fill requirement, however, the test patterns aregrossly over-specified. These large test patterns require extensivetester memory to store and a considerable time to apply from the testerto a circuit-under-test. FIG. 1 is a block diagram of a conventionalsystem 18 for testing digital circuits with scan chains. Externalautomatic testing equipment (ATE), or tester, 20 applies a set of fullyspecified test patterns 22 one by one to a CUT 24 in scan mode via scanchains 26 within the circuit. The circuit is then run in normal modeusing the test pattern as input, and the test response to the testpattern is stored in the scan chains. With the circuit again in scanmode, the response is then routed to the tester 20, which compares theresponse with a fault-free reference response 28, also one by one. Forlarge circuits, this approach becomes infeasible because of large testset sizes and long test application times. It has been reported that thevolume of test data can exceed one kilobit per single logic gate in alarge design. The significant limitation of this approach is that itrequires an expensive, memory-intensive tester and a long test time totest a complex circuit.

These limitations of time and storage can be overcome to some extent byadopting a built-in self-test (BIST) framework, as shown in the U.S.Pat. No. 4,503,537 and FIG. 13. In BIST, additional on-chip circuitry isincluded to generate test patterns, evaluate test responses, and controlthe test. For example, a pseudo-random pattern generator 121 is used togenerate the test patterns, instead of having deterministic testpatterns. Additionally, a multiple input signature register (MISR) 122is used to generate and store a resulting signature from test responses.In conventional logic BIST, where pseudo-random patterns are used astest patterns, 95-96% coverage of stuck-at faults can be achievedprovided that test points are employed to address random-patternresistant faults. On average, one to two test points may be required forevery 1000 gates. In BIST, all responses propagating to observableoutputs and the signature register have to be known. Unknown valuescorrupt the signature and therefore must be bounded by additional testlogic. Even though pseudo-random test patterns appear to cover asignificant percentage of stuck-at faults, these patterns must besupplemented by deterministic patterns that target the remaining, randompattern resistant faults. Very often the tester memory required to storethe supplemental patterns in BIST exceeds 50% of the memory required inthe deterministic approach described above. Another limitation of BISTis that other types of faults, such as transition or path delay faults,are not handled efficiently by pseudo-random patterns. Because of thecomplexity of the circuits and the limitations inherent in BIST, it isextremely difficult, if not impossible, to provide a set of specifiedtest patterns that fully covers hard-to-test faults.

Weighted pseudo-random testing is another method that is used to addressthe issue of the random pattern resistant faults. In principle, thisapproach expands the pseudo-random test pattern generators by biasingthe probabilities of the input bits so that the tests needed forhard-to-test faults are more likely to occur. In general, however, acircuit may require a very large number of sets of weights, and, foreach weight set, a number of random patterns have to be applied. Thus,although the volume of test data is usually reduced in comparison tofully specified deterministic test patterns, the resultant testapplication time increases. Moreover, weighted pseudo-random testingstill leaves a fraction of the fault list left uncovered. Details ofweighted random pattern test systems and related methods can be found ina number of references including U.S. Pat. Nos. 4,687,988; 4,801,870;5,394,405; 5,414,716; and 5,612,963. Weighted random patterns have beenprimarily used as a solution to compress the test data on the tester.The generation hardware appears to be too complex to place it on thechip. Consequently, the voluminous test data is produced off-chip andmust pass through relatively slow tester channels to thecircuit-under-test. Effectively, the test application time can be muchlonger than that consumed by the conventional deterministic approachusing ATPG patterns.

Several methods to compress test data before transmitting it to thecircuit-under-test have been suggested. They are based on theobservation that the test cubes (i.e., the arrangement of test patternsbits as they are stored within the scan chains of a circuit-under-test)frequently feature a large number of unspecified (don't care) positions.One method, known as reseeding of linear feedback shift registers(LFSRs), was first proposed in B. Koenemann, “LFSR-Coded Test PatternsFor Scan Designs,” Proc. European Test Conference, pp. 237-242 (1991).Consider an n-bit LFSR with a fixed polynomial. Its output sequence isthen completely determined by the initial seed. Thus, applying thefeedback equations recursively provides a system of linear equationsdepending only on the seed variables. These equations can be associatedwith the successive positions of the LFSR output sequence. Consequently,a seed corresponding to the actual test pattern can be determined bysolving the system of linear equations, where each equation representsone of the specified positions in the test pattern. Loading theresultant seed into the LFSR and subsequently clocking it will producethe desired test pattern. A disadvantage of this approach, however, isthat seed, which encodes the contents of the test cube, is limited toapproximately the size of the LFSR. If the test cube has more specifiedpositions than the number of stages in LFSR, the test cube cannot beeasily encoded with a seed. Another disadvantage of this approach is thetime it requires. A tester cannot fill the LFSR with a seed concurrentlywith the LFSR generating a test pattern from the seed. Each of theseacts must be done at mutually exclusive times. This makes the operationof the tester very inefficient, i.e., when the seed is serially loadedto the LFSR the scan chains do not operate; and when the loading of thescan chains takes place, the tester cannot transfer a seed to the LFSR.

Another compression method is based on reseeding of multiple polynomialLFSRs (MP-LFSRs) as proposed in S. Hellebrand et al., “Built-In Test ForCircuits With Scan Based On Reseeding of Multiple Polynomial LinearFeedback Shift Registers,” IEEE Trans. On Computers, vol. C-44, pp.223-233 (1995). In this method, a concatenated group of test cubes isencoded with a number of bits specifying a seed and a polynomialidentifier. The content of the MP-LFSR is loaded for each test group andhas to be preserved during the decompression of each test cube withinthe group. The implementation of the decompressor involves adding extramemory elements to avoid overwriting the content of the MP-LFSR duringthe decompression of a group of test patterns. A similar technique hasbeen also discussed in S. Hellebrand et al., “Pattern generation for adeterministic BIST scheme,” Proc. ICCAD, pp. 88-94 (1995). Reseeding ofMP-LFSRs was further enhanced by adopting the concept of variable-lengthseeds as described in J. Rajski et al., “Decompression of test datausing variable-length seed LFSRs”, Proc. VLSI Test Symposium, pp.426-433 (1995) and in J. Rajski et al., “Test Data Decompression forMultiple Scan Designs with Boundary Scan”, IEEE Trans. on Computers,vol. C-47, pp. 1188-1200 (1998). This technique has a potential forsignificant improvement of test pattern encoding efficiency, even fortest cubes with highly varying number of specified positions. The samedocuments propose decompression techniques for circuits with multiplescan chains and mechanisms to load seeds into the decompressor structurethrough the boundary-scan. Although this scheme significantly improvesencoding capability, it still suffers from the two drawbacks notedabove: seed-length limitations and mutually exclusive times for loadingthe seed and generating test patterns therefrom.

The above reseeding methods thus suffer from the following limitations.First, the encoding capability of reseeding is limited by the length ofthe LFSR. In general, it is very difficult to encode a test cube thathas more specified positions than the length of the LFSR. Second, theloading of the seed and test pattern generation therefrom are done intwo separate, non-overlapping phases. This results in poor utilizationof the tester time.

A different attempt to reduce test application time and test data volumeis described in I. Hamzaoglu et al., “Reducing Test Application Time ForFull Scan Embedded Cores,” Proc. FTCS-29, pp. 260-267 (1999). Thisso-called parallel-serial full scan scheme divides the scan chain intomultiple partitions and shifts in the same test pattern to each scanchain through a single scan input. Clearly, a given test pattern mustnot contain contradictory values on corresponding cells in differentchains loaded through the same input. Although partially specified testcubes may allow such operations, the performance of this scheme stronglyrelies on the scan chain configuration, i.e., the number of the scanchains used and the assignment of the memory elements to the scanchains. In large circuits such a mapping is unlikely to assume anydesired form, and thus the solution is not easily scalable. Furthermore,a tester using this scheme must be able to handle test patterns ofdifferent scan chain lengths, a feature not common to many testers.

Further, some of the DFT techniques include compactors to compress thetest responses from the scan chains. There are generally two types ofcompactors: time compactors and spatial compactors. Time compactorstypically have a feedback structure with memory elements for storing asignature, which represents the results of the test. After the signatureis completed it is read and compared to a fault-free signature todetermine if an error exists in the integrated circuit. Spatialcompactors generally compress a collection of bits (called a vector)from scan chains. The compacted output is analyzed in real time as thetest responses are shifted out of the scan chains. Spatial compactorscan be customized for a given circuit under test to reduce the aliasingphenomenon, as shown in the U.S. Pat. No. 5,790,562 and in few otherworks based on multiplexed parity trees or nonlinear trees comprisingelementary gates such as AND, OR, NAND, and NOR gates.

Linear spatial compactors are built of Exclusive-OR (XOR) orExclusive-NOR (XNOR) gates to generate n test outputs from the m primaryoutputs of the circuit under test, where n<m. Linear compactors differfrom nonlinear compactors in that the output value of a linear compactorchanges with a change in just one input to the compactor. With nonlinearcompactors, a change in an input value may go undetected at the outputof the compactor. However, even linear compactors may mask errors in anintegrated circuit. For example, the basic characteristic an XOR(parity) tree is that any combination of odd number of errors on itsinputs propagates to their outputs, and any combination of even numberof errors remains undetected.

An ideal compaction algorithm has the following features: (1) it is easyto implement as a part of the on-chip test circuitry, (2) it is not alimiting factor with respect to test time, (3) it provides a logarithmiccompression of the test data, and (4) it does not lose informationconcerning faults. In general, however, there is no known compactionalgorithm that satisfies all the above criteria. In particular, it isdifficult to ensure that the compressed output obtained from a faultycircuit is not the same as that of the fault-free circuit. Thisphenomenon is often referred to as error masking or aliasing and ismeasured in terms of the likelihood of its occurrence. An example oferror masking occurs when the spatial compactor reads two fault effectsat the same time. The multiple fault effects cancel each other out andthe compactor output is the same as if no faults occurred.

Unknown states are also problematic for error detection. An unknownstate on one or more inputs of an XOR tree generates unknown values onits output, and consequently masks propagation of faults on otherinputs. A common application of space compactors is to combine theobservation points inserted into the CUT as a part ofdesign-for-testability methodology. The spatial compactors can be alsoused to reduce the size of the time compactors by limiting the number oftheir parallel inputs.

Undoubtedly, the most popular time compactors used in practice arelinear feedback shift registers (LFSRs). In its basic form, the LFSR(see FIG. 14) is modified to accept an external input in order to act asa polynomial divider. An alternative implementation (called type IILFSR) is shown in FIG. 15. The input sequence, represented by apolynomial, is divided by the characteristic polynomial of the LFSR. Asthe division proceeds, the quotient sequence appears at the output ofthe LFSR and the remainder is kept in the LFSR. Once testing iscompleted, the content of the LFSR can be treated as a signature.

FIG. 16 shows another time compactor (which is a natural extension ofthe LFSR-based compactor) called a multiple-input LFSR, also known as amultiple-input signature register (MISR). The MISR is used to testcircuits in the multiple scan chain environment such as shown in theU.S. Pat. No. 4,503,537. MISRs feature a number of XOR gates added tothe flip-flops. The CUT scan chain outputs are then connected to thesegates.

FIG. 17 shows an example of a pipelined spatial compactor with a bank offlip-flops separating stages of XOR gates. A clock (not shown) controlsthe flip-flops and allows a one-cycle delay before reading the compactedoutput.

The limitation of spatial compactors, such as the one shown in FIG. 17,is that unknown states can reduce fault coverage. Time compactors, suchas shown in FIGS. 14, 15, and 16, are completely unable to handleunknown states since an unknown state on any input can corrupt thecompressed output generated by the compactor. With both time compactorsand spatial compactors, multiple fault effects can reduce faultcoverage. Additionally, if a fault effect is detected within theintegrated circuit, these compactors have limited ability to localizethe fault.

An object of the invention, therefore, is to provide an efficientcompactor that can select which scan chains are analyzed. This abilityto select allows the compactor to generate a valid compressed outputeven when receiving unknown states or multiple fault effects on itsinputs. The compactor can also be used diagnostically to determine thelocation of faults within an integrated circuit.

SUMMARY

A method according to the invention for applying test patterns to scanchains in a circuit-under-test includes providing a compressed testpattern of bits; decompressing the compressed test pattern into adecompressed test pattern of bits as the compressed test pattern isbeing provided; and applying the decompressed test pattern to scanchains of the circuit-under-test. If desired, the method may furtherinclude applying the decompressed test pattern to scan chains of thecircuit-under-test as the compressed test pattern is being provided.

The method may also include providing the compressed test pattern,decompressing the compressed test pattern, and applying the decompressedpattern synchronously. These acts may be performed at a same clock rate.Alternatively, the compressed test pattern may be provided at a lowerclock rate and the compressed test pattern decompressed and applied at ahigher clock rate. In yet another alternative, the compressed patternmay be provided and decompressed at a higher clock rate and thedecompressed pattern applied at a lower clock rate.

Decompressing the compressed test pattern may comprise generating duringa time period a greater number of decompressed test pattern bits thanthe number of compressed test pattern bits provided during the same timeperiod. One way the greater number of bits may be generated is byproviding a greater number of outputs for decompressed test pattern bitsthan the number of inputs to which the compressed test pattern bits areprovided. Another way the greater number of bits may be generated is bygenerating the decompressed test pattern bits at a higher clock ratethan the clock rate at which the compressed test pattern bits areprovided.

Decompressing the compressed test pattern may further comprisegenerating each bit of the decompressed pattern by logically combiningtwo or more bits of the compressed test pattern. This logicallycombining may include combining the bits with an XOR operation, an XNORoperation or a combination of the two operations.

In one embodiment of the invention, the providing and decompressingoccur within the circuit-under-test. In another embodiment of theinvention, the providing and decompressing occur within a tester, thetester applying the decompressed test pattern to scan chains of thecircuit-under-test.

A circuit according to the invention may comprise a decompressor,circuit logic, and scan chains for testing the circuit logic. Thedecompressor is adapted to receive a compressed test pattern of bits anddecompress the test pattern into a decompressed test pattern of bits asthe compressed test pattern is being received. The scan chains arecoupled to the decompressor and are adapted to receive the decompressedtest pattern. The decompressor may comprise a linear finite statemachine adapted to receive the compressed test pattern.

A tester according to the invention may comprise storage, adecompressor, and one or more tester channels. The storage is adapted tostore a set of compressed test patterns of bits. The decompressor iscoupled to the storage and adapted to receive a compressed test patternof bits provided from the storage and to decompress the test patterninto a decompressed test pattern of bits as the compressed test patternis being received. The tester channels are coupled to the decompressorand adapted to receive a decompressed test pattern and apply thedecompressed test pattern to a circuit-under-test.

In another embodiment, a compactor is disclosed that selects testresponses in one or more scan chains to compact into a compressedoutput, while one or more other test responses are masked. Thus, testresponses containing unknown states may be masked to ensure that thecompactor generates a valid compressed output. Additionally, testresponses can be masked to ensure fault masking does not occur. Thecompactor can also analyze test responses from individual scan chains todiagnostically localize faults in an integrated circuit.

A compactor includes selection circuitry that controls which scan chainsare analyzed. The selection circuitry passes desired test responses fromscan chains onto a compactor, while masking other test responses. In oneembodiment, the selection circuitry may include an identificationregister that is loaded with a unique identifier of a scan chain. Basedon the state of a flag register, either only the test response storedwithin the scan chain identified is passed to the compactor or all testresponses are passed to the compactor except the test responseassociated with the identified scan chain.

In another embodiment, the selection circuitry includes a flag thatcontrols whether only selected test responses are compacted or whetherall test responses are compacted.

In yet another embodiment, a control register is used that individuallyidentifies each scan chain included in compaction. In this embodiment, avariable number (e.g., 1, 2, 3, 4 . . . ) of test responses within scanchains may be included in compaction. Alternatively, the controlregister may store a unique identifier that is decoded to select onetest response that is compacted.

In still another embodiment, the selection circuitry includes a controlline that masks bits from scan chains on a per clock-cycle basis.Consequently, a test response may have only individual bits masked whilethe remaining bits of the test response are compacted.

These and other aspects and features of the invention are describedbelow with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional system for testing digitalcircuits with scan chains.

FIG. 2 is a block diagram of a test system according to the inventionfor testing digital circuits with scan chains.

FIG. 3 is a block diagram of a second embodiment of a system accordingto the invention for testing digital circuits with scan chains.

FIGS. 4A-B are block diagrams of a test system according to theinvention include timing diagrams illustrating different possible timingrelationships possible between the components of the system.

FIG. 5 is a block diagram of a decompressor according to the invention,including a linear finite state machine (LFSM) and phase shifter.

FIG. 6 shows in more detail a first embodiment of the decompressor ofFIG. 5 coupled to a scan chain.

FIG. 7 shows the logical expressions for the bits stored in each scancell in the scan chain of FIG. 5

FIGS. 8A-8D illustrate alternative embodiments of the LFSM of FIG. 5.

FIG. 9 illustrates a preferred embodiment of a 32-bit LFSM

FIG. 10 illustrates an alternative embodiment of the phase shifter ofFIG. 5.

FIG. 11 illustrates the use of parallel-to-serial conversion forapplying a compressed test pattern to the decompressor.

FIG. 12 is a block diagram of a tester according to the invention fortesting digital circuits with scan chains.

FIG. 13 is a block diagram of a prior art system using a built-in-testsystem.

FIG. 14 is a circuit diagram of a prior art type I LFSR compactor.

FIG. 15 is a circuit diagram of a prior art type II LFSR compactor.

FIG. 16 is a circuit diagram of a prior art architecture of a multipleinput signature register (MISR) compactor shown receiving input fromscan chains.

FIG. 17 is a circuit diagram of a prior art pipelined spatial compactor.

FIG. 18 is a block diagram of a selective compactor according to theinvention.

FIG. 19 shows one embodiment of a selective compactor, includingselection circuitry and a spatial compactor, for masking test responsesfrom scan chains.

FIG. 20 is another embodiment of a selective compactor includingselection circuitry and a time compactor for masking test responses fromscan chains.

FIG. 21 is yet another embodiment of a selective compactor includingselection circuitry and a cascaded compactor for masking individual bitsof test responses from scan chains.

FIG. 22 is another embodiment of a selective compactor includingselection circuitry and multiple compactors for masking test responses.

FIG. 23 is another embodiment of a selective compactor with selectioncircuitry that masks any variable number of test responses from the scanchains.

FIG. 24 is another embodiment of a selective compactor with programmableselection of scan chains.

FIG. 25 is a flowchart of a method for selectively compacting testresponses from scan chains.

DETAILED DESCRIPTION

Continuous Application and Decompression of Test Patterns to aCircuit-Under-Test

FIG. 2 is a block diagram of a system 30 according to the invention fortesting digital circuits with scan chains. The system includes a tester21 such as external automatic testing equipment (ATE) and a circuit 34that includes as all or part of it a circuit-under-test (CUT) 24. Thetester 21 provides from storage a set of compressed test patterns 32 ofbits, one pattern at a time, through tester channels 40 to the circuit34 such as an IC. A compressed pattern, as will be described, containsfar fewer bits than a conventional uncompressed test pattern. Acompressed pattern need contain only enough information to recreatedeterministically specified bits. Consequently, a compressed pattern istypically 2% to 5% of the size of a conventional test pattern andrequires much less tester memory for storage than conventional patterns.As importantly, compressed test patterns require much less time totransmit from a tester to a CUT 24.

Unlike in the prior reseeding techniques described above, the compressedtest patterns 32 are continuously provided from the tester 21 to scanchains 26 within the CUT 24 without interruption. As the compressed testpattern is being provided by the tester 21 to the input channels of adecompressor 36 within the circuit 34, the decompressor decompresses thecompressed pattern into a decompressed pattern of bits. The decompressedtest pattern is then applied to the scan chains 26. This application ispreferably done while the compressed test pattern is being provided tothe circuit 34, but it need not be so. After circuit logic within theCUT 24 is clocked with a decompressed test pattern in the scan chains26, the test response to that pattern is captured in the scan chains andtransmitted to the tester 21 for comparison with the compressedfault-free reference responses 41 stored therein.

In a typical configuration, the decompressor 36 has one output per scanchain 26, and there are more scan chains than input channels to thedecompressor. However, as will be described, other configurations arealso possible in which the decompressor outputs are fewer than or equalto the input channels. The decompressor generates in a given time perioda greater number of decompressed bits at its outputs than the number ofcompressed pattern bits it receives during the same time period. This isthe act of decompression, whereby the decompressor 36 generates agreater number of bits than are provided to it in a given time period.

To reduce the data volume of the test response and the time for sendingthe response to the tester, the circuit 34 can include means forcompressing the test response that is read from the scan chains 26. Onestructure for providing such compression is one or more spatialcompactors 38. The compressed test responses produced by the compactors38 are then compared one by one with compressed reference responses 40.A fault is detected if a reference response does not match an actualresponse. FIG. 3 shows another structure that can be used forcompressing the test response. A multiple input signature register(MISR) 42 compresses multiple test pattern responses into a signaturethat is then sent to the tester. There it is compared to a referencesignature 44. Compacting the test response in the above ways isdesirable but not necessary to the present decompression method andsystem.

The providing of a compressed test pattern to a circuit, itsdecompression into a decompressed test pattern, and the application ofthe decompressed test pattern to the scan chains is performedsynchronously, continuously, and substantially concurrently. The rate atwhich each act occurs, however, can vary. All acts can be performedsynchronously at a same clock rate if desired. Or the acts can beperformed at different clock rates. If the acts are performed at thesame clock rate, or if the compressed test patterns are provided anddecompressed at a higher clock rate than at which the decompressed testpatterns are applied to the scan chains, then the number of outputs ofdecompressor 36 and associated scan chains will exceed the number ofinput channels of the decompressor, as in FIG. 2. In this first case,decompression is achieved by providing more decompressor outputs thaninput channels. If the compressed test patterns are provided at a lowerclock rate and decompressed and applied to the scan chains at a higherclock rate, then the number of outputs and associated scan chains can bethe same, fewer, or greater than the number of input channels. In thissecond case, decompression is achieved by generating the decompressedtest pattern bits at a higher clock rate than the clock rate at whichthe compressed test pattern bits are provided.

FIG. 4A illustrates an embodiment of the first case in which thecompressed pattern is provided and decompressed at a higher clock rateand the decompressed pattern is applied synchronously to the scan chainsat a lower clock rate. The tester 21 provides the bits of the compressedpattern through a tester channel 40 to an input channel 37 of thedecompressor 36 at a higher rate set by clock 0 (C0). The decompressoris clocked by clock 1 (C1) at the same rate as the tester and producesat outputs 39 the bits of the decompressed pattern at that rate. Thesedecompressed bits, however, are applied to the scan chains 26 at a lowerrate set by clock 2 (C2), which clocks the bits into the scan chains.This difference in rates is illustrated in the exemplary timing diagramin FIG. 4A (the actual difference can be much greater). Because of thedifference therein, only every other output of the decompressor iswritten to the scan chains. But that is taken into account in theinitial test pattern generation. One advantage of clocking the tester,decompressor, and scan chains as shown is that the tester requires fewerchannels than the number of scan chains to provide the test pattern tothe CUT 24. By clocking the tester at a higher clock rate C0, the timerequired to apply the compressed test pattern to the circuit 34 issignificantly reduced. Another advantage is in low power applications,where the power dissipated during test mode has to be controlled. Thiscan be done by reducing the clock rate C2 at which bits are shifted intothe scan chains.

FIG. 4B illustrates an embodiment of the second case in which thecompressed test pattern is provided at a lower clock rate anddecompressed and applied synchronously at a higher clock rate. Here, thetester 21 provides the bits of the compressed pattern through channels40 to the input channels 37 of the decompressor 36 at a lower rate setby clock 0 (C0). The decompressor is clocked by clock 1 (C1) at a higherrate. The decompressed bits are applied through its outputs 39 to thescan chains 26 by clock 2 (C2) at the same rate as clock 1. Thisdifference in rates is illustrated in the exemplary timing diagram inFIG. 4B (the actual difference can be much greater). Because of thedifference, the decompressor 36 reads the same bits from the tester 21twice before they change. The decompressor, however, includes a statemachine, as will be described, and its outputs change each clock cyclebecause its internal states change. One advantage of clocking thetester, decompressor, and scan chains as shown in FIG. 4B is that onecan utilize a tester 21 that has many channels but with little memorybehind them. By providing bits on more tester channels per clock cycle,the lack of memory depth is overcome and the time required for applyingthe compressed test pattern is reduced.

FIG. 5 is a block diagram of a decompressor according to the invention.In a preferred embodiment, decompressor 36 comprises a linear finitestate machine (LFSM) 46 coupled, if desired, through its taps 48 to aphase shifter 50. The LFSM through the phase shifter provides highlylinearly independent test patterns to the inputs of numerous scan chainsin the CUT 24. The LFSM can be built on the basis of the canonical formsof linear feedback shift registers, cellular automata, or transformedLFSRs that can be obtained by applying a number of m-sequence preservingtransformations. The output of the LFSM is applied to the phase shifter,which ensures that the decompressed pattern bits present within each ofthe multiple scan chains 26 at any given time do not overlap in pattern(i.e., are out of phase).

The concept of continuous flow decompression described herein rests onthe fact noted above that deterministic test patterns typically haveonly between 2 to 5% of bits deterministically specified, with theremaining bits randomly filled during test pattern generation. (Testpatterns with partially specified bit positions are called test cubes,an example of which appears in Table 2.) These partially specified testcubes are compressed so that the test data volume that has to be storedexternally is significantly reduced. The fewer the number of specifiedbits in a test cube, the better is the ability to encode the informationinto a compressed pattern. The ability to encode test cubes into acompressed pattern is exploited by having a few decompressor inputchannels driving the circuit-under-test, which are viewed by the testeras virtual scan chains. The actual CUT 24, however, has its memoryelements connected into a large number of real scan chains. Under thesecircumstances, even a low-cost tester that has few scan channels andsufficiently small memory for storing test data can drive the circuitexternally.

FIG. 6 shows in more detail a first embodiment of the decompressor ofFIG. 5. The LFSM is embodied in an eight stage Type 1 LFSR 52implementing primitive polynomial h(x)=x⁸+x⁴+x³+x²+1. The phase shifter50, embodied in a number of XOR gates, drives eight scan chains 26, eacheight bits long. The structure of the phase shifter is selected in sucha way that a mutual separation between its output channels C0-C7 is atleast eight bits, and all output channels are driven by 3-input (tap)XOR functions having the following forms:

TABLE 1 C₀ = s₄ ⊕ s₃ ⊕ s₁ C₄ = s₄ ⊕ s₂ ⊕ s₁ C₁ = s₇ ⊕ s₆ ⊕ s₅ C₅ = s₅ ⊕s₂ ⊕ s₀ C₂ = s₇ ⊕ s₃ ⊕ s₂ C₆ = s₆ ⊕ s₅ ⊕ s₃ C₃ = s₆ ⊕ s₁ ⊕ s₀ C₇ = s₇ ⊕s₂ ⊕ s₀where C_(i) is the ith output channel and s_(k) indicates the kth stageof the LFSR. Assume that the LFSR is fed every clock cycle through itstwo input channels 37 a, 37 b and input injectors 48 a, 48 b (XOR gates)to the second and the sixth stages of the register. The input variables“a” (compressed test pattern bits) received on channel 37 a are labeledwith even subscripts (a₀, a₂, a₄, . . . ) and the variables “a” receivedon channel 37 b are labeled with odd subscripts (a₁, a₃, a₅, . . . ).Treating these external variables as Boolean, all scan cells can beconceptually filled with symbolic expressions being linear functions ofinput variables injected by tester 21 into the LFSR 52. Given thefeedback polynomial, the phase shifter 50, the location of injectors 48a, b as well as an additional initial period of four clock cycles duringwhich only the LFSR is supplied by test data, the contents of each scancell within the scan chains 26 in FIG. 6 can be logically determined.FIG. 7 gives the expressions for the 64 scan cells in FIG. 6, with thescan chains numbered 0 through 7 in FIG. 6 corresponding to the scanchains C7, C1, C6, . . . identified in FIG. 6. The expressions for eachscan chain in FIG. 7 are listed in the order in which the information isshifted into the chain, i.e., the topmost expression represents the datashifted in first.

Assume that the decompressor 36 in FIG. 6 is to generate a test patternbased on the following partially specified test cube in Table 2 (thecontents of the eight scan chains are shown here horizontally, with theleftmost column representing the information that is shifted first intothe scan chains):

TABLE 2 x x x x x x x x scan chain 0 x x x x x x x x scan chain 1 x x xx 1 1 x x scan chain 2 x x 0 x x x 1 x scan chain 3 x x x x 0 x x 1 scanchain 4 x x 0 x 0 x x x scan chain 5 x x 1 x 1 x x x scan chain 6 x x xx x x x x scan chain 7The variable x denotes a “don't care” condition. Then a correspondingcompressed test pattern can be determined by solving the followingsystem of ten equations from FIG. 7 using any of a number of well-knowntechniques such as Gauss-Jordan elimination techniques. The selectedequations correspond to the deterministically specified bits:

TABLE 3 a₂ ⊕ a₆ ⊕ a₁₁ = 1 a₀ ⊕ a₁ ⊕ a₄ ⊕ a₈ ⊕ a₁₃ = 1 a₄ ⊕ a₅ ⊕ a₉ ⊕ a₁₁= 0 a₀ ⊕ a₂ ⊕ a₅ ⊕ a₁₂ ⊕ a₁₃ ⊕ a₁₇ ⊕ a₁₉ = 1 a₁ ⊕ a₂ ⊕ a₄ ⊕ a₅ ⊕ a₆ ⊕ a₈⊕ a₁₂ ⊕ a₁₅ = 0 a₀ ⊕ a₁ ⊕ a₃ ⊕ a₅ ⊕ a₇ ⊕ a₈ ⊕ a₁₀ ⊕ a₁₁ ⊕ a₁₂ ⊕ a₁₄ ⊕a₁₈ ⊕ a₂₁ = 1 a₂ ⊕ a₃ ⊕ a₄ ⊕ a₉ ⊕ a₁₀ = 0 a₀ ⊕ a₁ ⊕ a₂ ⊕ a₆ ⊕ a₇ ⊕ a₈ ⊕a₁₃ ⊕ a₁₄ = 0 a₃ ⊕ a₄ ⊕ a₅ ⊕ a₆ ⊕ a₁₀ = 1 a₀ ⊕ a₁ ⊕ a₃ ⊕ a₇ ⊕ a₈ ⊕ a₉ ⊕a₁₀ ⊕ a₁₄ = 1It can be verified that the resulting seed variables a₀, a₁, a₂, a₃ anda₁₃ are equal to the value of one while the remaining variables assumethe value of zero. This seed will subsequently produce a fully specifiedtest pattern in the following form (the initial specified positions areunderlined):

TABLE 4 1 0 1 0 0 1 0 0 1 1 0 0 0 1 0 0 1 1 1 1 1 1 1 0 0 0 0 1 0 0 1 11 0 1 0 0 0 0 1 1 1 0 1 0 0 0 0 1 1 1 1 1 1 1 1 0 1 0 0 1 1 0 0As can be observed, the achieved compression ratio (defined as thenumber of scan cells divided by the number of compressed pattern bits)is 64/(2×8+2×4)≈2.66. The fully specified test pattern is thencompressed into a compressed pattern of bits using any of a number ofknown methods.

FIGS. 8A-D illustrate various embodiments for the LFSM 46 of FIG. 5.FIG. 8A is a Type I LFSR 60. FIG. 8B is a Type II LFSR 62. FIG. 8C is atransformed LFSR 64. And FIG. 8D is a cellular automaton 66. All of themimplement primitive polynomials. Except for the cellular automaton 66,in each case the LFSM includes a number of memory elements connected ina shift register configuration. In addition, there are several feedbackconnections between various memory cells that uniquely determine thenext state of the LFSM. The feedback connections are assimilated intothe design by introducing injectors in the form of XOR gates near thedestination memory elements. The input channels 37 provide the bits of acompressed pattern to the LFSM through input injectors 48 a, b. Theinjectors are handled similarly as the other feedback connections withinthe LFSM except that their sources of bits are the input channels. Theinput channels 37 may have multiple fan-outs driving different LFSMinjectors 48 to improve the encoding efficiency.

FIG. 9 shows a preferred embodiment of a 32-bit LFSM in the form of are-timed LFSR 68. The injectors are spaced equally so that the inputvariables are distributed optimally once they are injected into theLFSM. In practice, the size of the LFSM depends on the number of realscan chains in a circuit, the desired compression ratio of encoding, andon certain structural properties of the circuit-under-test.

FIG. 10 illustrates an alternative embodiment of a phase shifter 50,constructed with an array of XNOR gates rather than XOR gates. Phaseshifters can be constructed with combinations of XNOR and XOR gates aswell.

FIG. 11 illustrates the use of parallel-to-serial conversion forapplying a compressed test pattern to the decompressor. If the inputchannels 37 to the decompressor 36 are fewer in number than the numberof channels 40 of the tester 21, it can be advantageous to provide aparallel-to-serial converter such as registers 70 at the input to thedecompressor. The registers 70 are clocked such that their contents areshifted out before the next set of bits is applied to the register fromthe tester 21. The continuous flow of the test patterns is thuspreserved.

FIG. 12 is a block diagram of a tester 21 embodiment that includes thedecompressor 36, rather than providing it in the circuit 34. The testerdecompresses the test pattern internally and transmits the decompressedtest pattern to the CUT 24. Such a tester has advantages where testingtime is not as critical and it is preferred not to add a decompressor toeach circuit-under-test. Storage requirements are still reduced becausecompressed test patterns (rather than full test patterns) need only bestored. In addition, in a variation of the above tester embodiment, thecompactors 38 can also be included in the tester 21 rather than thecircuit 34. The circuit then returns uncompressed test responses to thetester. This further simplifies the circuit's design.

The process of decompressing a test pattern will now be described inmore detail, with reference to FIG. 5. The LFSM 46 starts its operationfrom an initial all-zero state. Assuming an n-bit LFSM and m inputinjectors, ┌n/m┐ clock cycles may be used to initialize the LFSM beforeit starts generating bits corresponding to the actual test patterns.After initialization of the LFSM and assuming clocks C0 and C1 arerunning at the same rate, a new bit is loaded in parallel into each scanchain 26 every clock cycle via the phase shifter 50. At this time, thecircuit-under-test 34 is operated in the scan mode, so that thedecompressed test pattern fills the scan chains 26 with 0s and 1s (andshifts out any previous test response stored there). A small number ofbit positions in the scan chains, therefore, get deterministicallyspecified values while the remaining positions are filled with randombits generated by the LFSM. The number of clock cycles for which a testpattern is shifted is determined by the length of the longest scan chainwithin the circuit, the number being at least as great as the number ofcells in the longest scan chain. A scan-shift signal is therefore heldhigh for all the scan chains until the longest scan chain gets theentire test pattern. The shorter scan chains in the circuit are leftjustified so that the first few bits that are shifted are overwrittenwithout any loss of information.

Patterns from the LFSM may be linearly dependent. In other words, it ispossible to determine various bit positions within the two-dimensionalstructure of multiple scan chains that are significantly correlated.This causes testability problems, as it is often not possible to providethe necessary stimulus for fault excitation to the gates driven bypositions that have some form of dependency between them. Consequently,the phase shifter 50 (such as an array of XOR gates or XNOR gates) maybe employed at the taps (outputs) of the LFSM to reduce lineardependencies between various bit positions within the scan chains. TheXOR logic can be two-level or multi-level depending on the size of theXOR gates. Every scan chain in the CUT 24 is driven by signals that areobtained by XOR-ing a subset of taps 48 from the LFSM. These taps aredetermined so that the encoding efficiency of the test cubes is stillpreserved. In addition, the taps are selected in a manner so that allmemory cells in the LFSM have approximately equal number of fan-outsignals and the propagation delays are suitably optimized. Once adecompressed test pattern is completely loaded into the scan chainsduring test mode, the CUT 24 is switched to the normal mode ofoperation. The CUT then performs its normal operation under the stimulusprovided by the test pattern in the scan chains. The test response ofthe CUT is captured in the scan chains. During the capture the LFSM isreset to the all-zero state before a new initialization cycle begins forloading the next test pattern.

Selectively Compacting Test Responses

FIG. 18 shows a block diagram of an integrated circuit 124 that includesmultiple scan chains 126 in a circuit under test 128. A selectivecompactor 130 is coupled to the scan chains 126 and includes a selectorcircuit 132 and a compactor 136. The illustrated system is adeterministic test environment because the scan chains 126 are loadedwith predetermined test patterns from an ATE (not shown). The testpatterns are applied to the core logic of the integrated circuit togenerate test responses, which are also stored in the scan chains 126(each scan chain contains a test response). The test responses containinformation associated with faults in the core logic of the integratedcircuit 124. Unfortunately, the test responses may also contain unknownstates and/or multiple fault effects, which can negatively impact theeffective coverage of the test responses. For example, if a memory cellis not initialized, it may propagate an unknown state to the testresponse. The test responses are passed to the selector circuit 132 ofthe selective compactor 130. The selector circuit 132 includes controllogic 134 that controls which of the test responses are passed throughthe selector circuit to the compactor 136. The control logic 134 cancontrol the selector circuit 132 such that test responses with unknownstates or multiple fault effects are masked. The control logic iscontrolled by one or more control lines. Although not shown, the controllines may be connected directly to a channel of an ATE or they may beconnected to other logic within the integrated circuit. For example, thecontrol lines may be coupled to a Linear Finite State Machine (e.g.,LSFR type 1, LSFR type 2, cellular automata, etc.) in combination with aphase shifter. The compactor 136 receives the desired test responsesfrom the selector circuit 132 and compacts the responses into acompressed output for analysis. The compressed output is comparedagainst a desired output to determine if the circuit under test containsany faults. The selection circuitry, compactor, and circuit under testare all shown within a single integrated circuit. However, the selectioncircuitry and compactor may be located externally of the integratedcircuit, such as within the ATE.

FIG. 19 shows one example of an integrated circuit 140 that includes aselective compactor 142 coupled to multiple scan chains 144 within acircuit under test. Although only 8 scan chains are shown, the testcircuit 140 may contain any number of scan chains. The selectivecompactor 142 includes a selector circuit 146 and a compactor 148. Thecompactor 148 is a linear spatial compactor, but any conventionalparallel test-response compaction scheme can be used with the selectorcircuit 146, as further described below. The selector circuit 146includes control logic 150, which includes an input register 152, shownin this example as a shift register. The input register 152 has a clockinput 154 and a data input 156. Each cycle of a clock on the clock input154, data from data input 156 shifts into the input register 152. Theregister 152 has multiple fields including a scan identification field158, a “one/not one” field 160 and a “not all/all” field 162. A controlregister 164 has corresponding bit positions to input register 152, andupon receiving an update signal on an update line 166, the controlregister 164 loads each bit position from input register 152 inparallel. Thus, the control register 164 also contains fields 158, 160,and 162. Although the control register 164 is shown generically as ashift register, the update line 166 is actually a control line to amultiplexer (not shown) that allows each bit position in register 164 toreload its own data on each clock cycle when the update linedeactivated. When the update line is activated, the multiplexer passesthe contents of register 152 to corresponding bit positions of thecontrol register 164. The control register 164 is then loadedsynchronously with the clock.

The selector circuit 146 includes logic gates, shown generally at 168,coupled to the control register 164. The logic gates 168 are responsiveto the different fields 158, 160, 162 of the control register 164. Forexample, the scan identification field 158 contains a sufficient numberof bits to uniquely identify any of the scan chains 144. The scanidentification field 158 of the control register 164 is connected to adecoder, shown at 170 as a series of AND gates and inverters. Thedecoder 170 provides a logic one on a decoder output depending on thescan identification field, while the other outputs of the decoder are alogic zero.

The one/not one field 160 of the control register 164 is used to eitherpass only one test response associated with the scan chain identified inthe scan identification field 158 or pass all of the test responsesexcept for the scan chain identified in the scan identification field.The all/not all field 162 is effectively an override of the otherfields. In particular, field 162 controls whether all of the testresponses in the scan chains 144 are passed to the compactor 148 or onlythe test responses as controlled by the scan identification field 158and the one/not one field 160. With field 162 cleared, only testresponses as controlled by the scan identification field 158 and field160 pass to the compactor 148. Conversely, if the field 162 is set to alogic one, then all of the test responses from all of the scan chains144 pass to the compactor 148 regardless of the scan identificationfield 58 and the one/not one field 160.

FIG. 20 shows another embodiment of a selective compactor 180 that iscoupled to scan chains 182. The selective compactor includes a selectorcircuit 184, which is identical to the selector circuit 146 described inrelation to FIG. 19. The selective compactor 180 also includes a timecompactor 184, which is well understood in the art to be a circularcompactor. The time compactor includes multiple flip-flops 186 and XORgates 188 coupled in series. A reset line 190 is coupled to theflip-flops 186 to reset the compactor 184. The reset line may be resetmultiple times while reading the scan chains. Output register 192provides a valid output of the compactor 84 upon activation of a readline 194.

Referring to both FIGS. 19 and 20, in operation the scan chains 182 areserially loaded with predetermined test patterns by shifting data onscan channels (not shown) from an ATE (not shown). Simultaneously, theinput register 152 is loaded with a scan identification and thecontrolling flags in fields 160, 162. The test patterns in the scanchains 144, 182 are applied to the circuit under test and test responsesare stored in the scan chains. Prior to shifting the test responses outof the scan chains, the update line 166 is activated, thus moving fields158, 160, 162 to the control register 164. The control register therebycontrols the logic gates 168 to select the test responses that arepassed to the compactors 148, 184. If the field 162 is in a state suchthat selection is not overridden, then certain of the test responses aremasked. In the example of FIG. 19, the spatial compactor 148 providesthe corresponding compressed output serially and simultaneously withshifting the test responses out of the scan chains. Conversely, in FIG.20 the selective compactor 180 does not provide the appropriatecompressed output until the read line 194 is activated. The selectivecompactor 180 provides a parallel compressed output as opposed toserial. The selective compactor 180 may be read multiple times (e.g.,every eighth clock cycle) while reading out the test responses.

FIG. 21 shows another embodiment of a selective compactor 200. Again,the selective compactor includes a selector circuit 202 and a compactor204. The compactor 204 is a type of spatial compactor called a cascadedcompactor. N scan chains 206 include M scan cells 208, each of whichstores one bit of the test response. The selector circuit 202 includeslogic gates 210, in this case shown as AND gates, coupled to a controlline 212. The compactor 204 is a time compactor with a single serialoutput 214. The control line 212 is used to mask the test responses. Inparticular, the control line 212 either masks all corresponding scancells in the scan chains or allows all of the scan cells to pass to thecompactor 180. The control line 212 operates to mask each column of scancells, rather than masking an entire scan chain. Thus, individual bitsfrom any scan chain can be masked on a per clock-cycle basis and theremaining bits of that scan chain applied to the compactor 204. Withcontrol line 212 activated, all bits from the scan chains pass to thecompactor. With control line 212 deactivated, all bits from the scanchains are masked. Although FIG. 21 shows only a single control line,additional control lines can be used to mask different groups of scanchains. Additionally, although control line 212 is shown as active high,it may be configured as active low.

FIG. 22 shows yet another embodiment of the selective compactor 220.Automated test equipment 222 provides test patterns to the scan chains224. The scan chains 224 are a part of the circuit under test 226. Thepatterns that are loaded into the scan chains 224 by the ATE are used todetect faults in the core logic of the circuit 226. The test responsesare stored in the scan chains 224 and are clocked in serial fashion tothe selective compactor 220. The selective compactor includes a selectorcircuit 228 and a compactor 230. The selector circuit 228 includescontrol logic including an input register 232, multiple controlregisters 234, 236, and multiple decoders 237 and 239. The register 232is loaded with a pattern of bits that are moved to the control registers234, 236 upon activation of an update line (not shown). The controlregisters 234, 236 are read by the decoders 237 and 239 and decoded toselect one or more logic gates 238. A flag 240 is used to override thedecoders 237 and 239 and pass all of the test responses to the compactor230. Although only a single flag 240 is shown, multiple flags may beused to separately control the decoders. In this example, the compactor230 includes multiple spatial compactors, such as compactors 242 and244. Each control register may be loaded with different data so that thecompactors 242, 244 can be controlled independently of each other.

FIG. 23 shows yet another embodiment of the present invention with aselective compactor 250. Control logic 252 variably controls which testresponses are masked and which test responses are compacted. Thus,activating the corresponding bit position in the control logic 252activates the corresponding logic gate associated with that bit andallows the test response to pass to the compactor. Conversely, any bitthat is not activated masks the corresponding test response.

FIG. 24 shows another embodiment of a selective compactor 256 includinga selector circuit 258 and compactor 260. In this case, an input shiftregister 262 having a bit position corresponding to each scan chain 264is used to selectively mask the scan chains. A clock is applied to line266 to serially move data applied on data line 268 into the shiftregister 262. At the appropriate time, an update line 265 is activatedto move the data from the shift register to a control register 269. Eachbit position that is activated in the control register 269 allows a testresponse from the scan chains 264 to pass to the compactor. All othertest responses are masked. Thus, the selective compactor can mask anyvariable number of test responses.

Each of the embodiments described above can be used as a diagnostic toolfor localizing faults in the circuit under test. For example, each testresponse can be analyzed individually by masking all other testresponses in the scan chains connected to the same compactor. By viewingthe test response individually, the bit position in the test responsecontaining fault effects can be determined.

FIG. 25 shows a flowchart of a method for selectively compacting testresponses. In process block 270, an ATE loads predetermined testpatterns into scan chains within an integrated circuit. This loading istypically accomplished by shifting the test patterns serially into thescan chains. The test patterns are applied to the circuit under test(process block 272) and the test responses are stored in the scan chains(process block 274). In process block 276, the selector circuit controlswhich test responses are masked. In particular, the selector circuitcontrols which scan chains are masked or which bits in the scan chainsare masked. For example, in FIG. 19, the selector circuit masks theentire scan chain that is identified in the scan identification field158. In FIG. 21, only individual bits of a scan chain are masked. In anyevent, in process block 276, the selector circuit typically masksunknown data or multiple fault effects so that the desired fault effectcan propagate to the output (in some modes of operation, all of the testresponses may pass to the output). In the event that the selectorcircuit includes a control register, the control register may be loadedconcurrently with loading the test patterns in the scan chains or it canbe loaded prior to reading the test responses. In process block 278, thetest responses (one or more of which have been masked) are passed to thecompactor and the compactor generates a compressed output associatedwith the test responses. In process block 280, the compressed outputgenerated by the compactor is compared to an ideal response. If theymatch, the integrated circuit is assumed to be fault free.

Having illustrated and described the principles of the illustratedembodiments, it will be apparent to those skilled in the art that theembodiments can be modified in arrangement and detail without departingfrom such principles. For example, any of the illustrated compactors canbe used with any of the illustrated selector circuits with minimummodification to create a selective compactor. Additionally, the selectorcircuit can easily be modified using different logic gates to achievethe selection functionality. For example, although the update lines areshown coupled to a separate bank of flip flops, the update lines caninstead be coupled to input registers having tri-state outputs forcontrolling the logic in the selector circuit. Still further, althoughthe scan chains are shown as serial shift registers, logic may be addedso as to output test response data in parallel to the selectivecompactor. Additionally, although multiple spatial and time compactorswere shown, compactors having features of both spatial and timecompactors may be used. Indeed, any conventional or newly developedcompactor may be used with the selection circuitry.

In view of the many possible embodiments to which the principles of theinvention may be applied, it should be understood that the illustrativeembodiment is intended to teach these principles and is not intended tobe a limitation on the scope of the invention. We therefore claim as ourinvention all that comes within the scope and spirit of the followingclaims and their equivalents.

We claim:
 1. A system, comprising: a circuit comprising a register and adecompressor, the decompressor comprising a linear feedback shiftregister (LFSR); and automatic testing equipment located external to thecircuit, wherein the register is configured to load compressed testpattern data from an output of the automatic testing equipment and tooutput the compressed test pattern data to an input logic gate of theLFSR, the input logic gate of the LFSR being configured to receive thecompressed test pattern data and logically combine the compressed testpattern data with data stored within the LFSR.
 2. The system of claim 1,wherein the register is configured to receive multiple bits of thecompressed test pattern data in parallel from multiple outputs of theautomatic testing equipment before the multiple bits of the compressedtest pattern data are output to the input logic gate of the LFSR.
 3. Thesystem of claim 1, wherein the decompressor further includes a phaseshifter having inputs coupled to outputs of the LFSR, the phase shifterbeing configured such that test pattern bits output from the phaseshifter are out of phase with one another.
 4. The system of claim 1,wherein the circuit further includes scan chains and wherein thedecompressor further includes a phase shifter coupled between the LFSRand the scan chains, the phase shifter being configured such that testpattern bits output from the phase shifter are out of phase with oneanother.
 5. The system of claim 1, wherein the register is configured toreceive the compressed test pattern data in parallel from multipleoutputs of the automatic testing equipment and shift the compressed testpattern into the LFSR.
 6. A method, comprising: loading a registerwithin a circuit with compressed test pattern bits; transferring thecompressed test pattern bits from the register to a decompressorcomprising a linear feedback shift register (LFSR); and decompressingthe compressed test pattern bits with the decompressor, wherein thedecompressing comprises logically combining the compressed test patternbits with bits stored within the decompressor.
 7. The method of claim 6,wherein the decompressor further comprises a phase shifter.
 8. Themethod of claim 6, wherein the decompressing occurs in the decompressoras the compressed test pattern bits are transferred from the register tothe decompressor.
 9. The method of claim 6, wherein the loadingcomprises loading the compressed test pattern bits into the registerfrom external automatic testing equipment (ATE).
 10. The method of claim6, further including loading scan chains with the decompressed testpattern bits.
 11. The method of claim 6, wherein the loading comprisesloading the compressed test pattern bits into the register in parallel,and wherein the transferring includes serially shifting the compressedtest pattern bits from the register into the decompressor.